For example, in an image forming device using an electrophotographic system, such as a facsimile, a printer or a copying machine, a toner is adhered to an electrostatic latent image formed on the surface of a photosensitive member to visualize it, the visualized image is transferred to a sheet or the like and thereafter it is fixed by being heated and pressurized. In terms of achieving high image quality and colorization, as a developer, a two-component developer containing a carrier and a toner is widely used.
In a development system using a two-component developer, a carrier and a toner are agitated and mixed within a development device, and the toner is charged by friction so as to have a predetermined amount. Then, the developer is supplied to a rotating development roller, a magnetic brush is formed on the development roller and the toner is electrically moved to the photosensitive member through the magnetic brush to visualize the electrostatic latent image on the photosensitive member. The carrier after the movement of the toner is left on the development roller, and is mixed again with the toner within the development device. Hence, as the properties of the carrier, a magnetic property for forming the magnetic brush, a charging property for providing a desired charge to the toner and durability for repeated use are required.
Hence, so-called coating carriers in which the surfaces of magnetic particles such as magnetite and various types of ferrite are coated with a resin are used more widely than ever. However, a coating resin layer on the surface of the particles wears out such as by long-term agitation within the development device. When the overall coating resin layer on the surface of the particles wears out, the charging property of the carrier is lowered and a charging failure of the toner occurs, with the result that the image quality is lowered.
Patent document 1 proposes that in order to enhance an adhesion strength to a coating resin layer, in ferrite particles having a specific composition, minute projections and recesses be formed in the surface of crystal grains. Patent document 2 also proposes that projections and recesses of a finely striped pattern be formed in the surface of ferrite particles.